Made header Spark log image as a link to History Server's top page.

Author: sarutak

core/src/main/scala/org/apache/spark/ui/UIUtils.scala

@@ -216,8 +216,10 @@ private[spark] object UIUtils extends Logging {
           <div class="row-fluid">
             <div class="span12">
               <h3 style="vertical-align: middle; display: inline-block;">
-                <img src={prependBaseUri("/static/spark-logo-77x50px-hd.png")}
-                     style="margin-right: 15px;" />
+                <a style="text-decoration: none" href ={prependBaseUri("/")}>
+                  <img src={prependBaseUri("/static/spark-logo-77x50px-hd.png")}
+                       style="margin-right: 15px;" />
+                </a>
                 {title}
               </h3>
             </div>